DTV broadcasting in Europe has employed coded orthogonal frequency-division multiplexing (COFDM) that employs a multiplicity of RF carrier waves closely spaced across each 8-MHz-wide television channel, rather than a single RF carrier wave per television channel. Adjacent carrier waves are orthogonal to each other. Successive multi-bit symbols are selected from a serial data stream and used to modulate respective ones of the multiplicity of RF carrier waves in turn, in accordance with a conventional modulation scheme—such as quaternary phase shift keying (QPSK) or quadrature amplitude modulation (QAM). QPSK is preferably DQPSK, using differential modulation that is inherently insensitive to slowly changing amplitude and phase distortion. DPSK simplifies carrier recovery in the receiver. Customarily, the QAM is either 16QAM or 64QAM using square 2-dimensional modulation constellations. In actual practice, the RF carrier waves are not modulated individually. Rather, a single carrier wave is modulated at high symbol rate using QPSK or QAM. The resulting modulated carrier wave is then transformed in a fast inverse discrete Fourier transform (I-DFT) procedure to generate the multiplicity of RF carrier waves each modulated at low symbol rate.
In Europe, broadcasting to hand-held receivers is done using a system referred to as DVB-H. DVB-H (Digital Video Broadcasting—Handheld) is a digital broadcast standard for the transmission of broadcast content to handheld receivers, published in 2004 by the European Telecommunications Standards Institute (ETSI) and identified as EN 302304. DVB-H, as a transmission standard, specifies the physical layer as well as the elements of the lower protocol layers. It uses a power-saving technique based on the time-multiplexed transmission of different services. The technique, called “time slicing”, allows substantial saving of battery power. Time slicing allows soft hand-over as the receiver moves from network cell to network cell. The relatively long power-save periods may be used to search for channels in neighboring radio cells offering the selected service. Accordingly, at the border between two cells, a channel hand-over can be performed that is imperceptible by the user. Both the monitoring of the services in adjacent cells and the reception of the selected service data can utilize the same front-end tuner.
In contrast to other DVB transmission systems, which are based on the DVB Transport Stream adopted from the MPEG-2 standard, the DVB-H system is based on Internet Protocol (IP). The DVB-H baseband interface is an IP interface allowing the DVB-H system to be combined with other IP-based networks. Even so, the MPEG-2 transport stream is still used by the base layer. The IP data are embedded into the transport stream using Multi-Protocol Encapsulation (MPE), an adaptation protocol defined in the DVB Data Broadcast Specification. At the MPE level, DVB-H employs an additional stage of forward error correction called MPE-FEC, which is essentially (255, 191) transverse Reed-Solomon (TRS) coding. The transverse direction is orthogonal to the direction of the “lateral” (204, 188) Reed-Solomon (RS) coding employed both in DVB-H and in DVB-T terrestrial broadcasting to stationary DTV receivers. This TRS coding reduces the S/N requirements for reception by a handheld device by a 7 dB margin compared to DVB-T. The block interleaver used for the TRS coding creates a specific frame structure, referred to as the “MPE-FEC frame” or simply the “FEC frame”, for incorporating the incoming data of the DVB-H codec.
The physical radio transmission of DVB-H is performed according to the DVB-T standard and employs OFDM multi-carrier modulation. DVB-T employed coded orthogonal frequency division multiplexing (COFDM) in which an 8-MHz-wide radio-frequency (RF) channel comprises somewhat fewer than 2000 or somewhat fewer than 8000 evenly-spaced carriers for transmitting to stationary DTV receivers. DVB-T2, an upgrade of DVB-T proposed in 2011, further permits somewhat fewer than 4000 evenly-spaced carrier waves better to accommodate transmitting to mobile receivers using DVB-H. These three choices as to number of carrier waves are commonly referred to as 2K, 8K and 4K options. DVB-H uses only a fraction (e.g., one quarter) of the digital payload capacity of the RF channel.
COFDM has been considered for DTV broadcasting in the United States of America (US), where 6-MHz-wide, rather than 8-MHz-wide, RF channels are employed for such broadcasting. The 2K, 8K and 4K options are retained in proposals for such DTV broadcasting, with bit rates being scaled back to suit 6-MHz-wide RF channels. COFDM of plural carrier waves may eventually supplant the 8-VSB amplitude-modulated single-carrier-wave system of DTV broadcasting used in the US at the time this specification was written. A driving force behind the adoption of COFDM for DTV broadcasting in the US is apt to be that its performance in single-frequency networks (SFNs) is superior to that of the 8-VSB AM single-carrier-wave system of broadcasting used in the US.
The DVB-T and DVB-H standards for European broadcasting employ Reed-Solomon (RS) coding followed by convolutional coding in the forward-error-correction (FEC) coding of DTV data. Decoding of the convolutional coding is effective in overcoming corruption caused by Johnson noise, which has additive white Gaussian noise (AWGN) characteristics, but occasionally decoding generates a running error. Subsequent RS coding after a byte de-interleave can suppress such running error and can also suppress burst errors in the COFDM demodulation results.
If COFDM is adopted for DTV broadcasting in the US, the convolutional coding used together with Reed-Solomon (RS) coding in the forward-error-correction (FEC) coding of DTV data is apt to be replaced by some form of coding that can be decoded using iterative soft-decision decoding procedures referred to as “turbo” decoding. Such forms of coding are commonly referred to as “turbo coding” and comprise parallel concatenated convolutional coding (PCCC), serial concatenated convolutional coding (SCCC), and product coding composed of concatenated block and convolutional coding. Low-density parity-check (LDPC) codes that are parallel concatenated provide another type of turbo coding that is decoded using iterative soft-decision decoding procedures. The iterative soft-decision decoding procedures used for turbo coding reduce errors in DTV data caused by additive white Gaussian noise (AWGN) significantly better than the decoding of simple convolutional coding can. This permits increasing the number of lattice points in the QAM symbol constellations used to modulate COFDM carriers by a factor of four, which exceeds the factor of 3/2 or 2 by which code rate is reduced by using turbo coding rather than simple convolutional coding. Accordingly, digital payload can be increased by a factor of 2 or 8/3 without sacrificing capability to withstand AWGN. Furthermore, turbo decoding is better able to overcome inter-symbol interference (ISI) than Viterbi decoding of simple convolutional coding can, which allows shortening of the COFDM guard interval and some additional increase in digital payload.
Turbo coding is less susceptible to running errors than simple convolutional coding because the component codes of the turbo coding exhibit temporal diversity between their respective coding algorithms. This phenomenon is referred to as “interleaver gain”, and the interleaving between the component codes of the turbo coding can be designed to obtain substantially as much interleaver gain as possible.
COFDM is able to overcome frequency-selective fading quite well, but reception will fail when there is severe flat-spectrum fading. Such flat-spectrum fading is sometimes referred to as a “drop-out” in received signal strength. Such drop-out occurs when the receiving site changes such that a sole effective signal transmission path is blocked by an intervening hill or structure, for example. Because the signaling rate in the individual OFDM carriers is very low, COFDM receivers are capable of maintaining reception despite drop-outs that are only a fraction of a second in duration. However, drop-outs that last as long as a few seconds disrupt television reception perceptibly. Such protracted drop-outs are encountered in a vehicular receiver when the vehicle passes through a tunnel, for example. By way of further example of a protracted drop-out in reception, a stationary DTV receiver may briefly discontinue COFDM reception when receiver synchronization is momentarily lost during dynamic multipath reception conditions, such as caused by aircraft flying over the reception site.
The ATSC standard directed to broadcasting digital television and digital data to M/H receivers used TRS coding that extended over eighty dispersed-in-time short time-slot intervals, rather than being confined to a single longer time-slot interval. A principal purpose of the TRS coding that extended over eighty time-slot intervals was overcoming occasional protracted drop-outs in received signal strength. Confining TRS coding to a single longer time-slot interval as done in DVB-H is advantageous, however, in that error-correction is completed within a shorter time. This helps speed up changes in RF-channel tuning, for example.
Iterative-diversity transmissions were proposed to ATSC to facilitate alternative or additional techniques for dealing with flat-spectrum fading of 8-VSB signals. Some of these proposals were directed to separate procedures being used for decoding earlier and later transmissions of the same coded data to generate respective sets of data packets, each identified after such decoding either as being probably correct or probably incorrect. Corresponding data packets from the two sets were compared, and a further set of data packets was chosen from the ones of the compared data packets more likely to be correct. A. L. R. Limberg proposed delaying earlier transmissions of concatenated convolutionally coded (CCC) data so as to be concurrently available with later transmissions of similar CCC data, then decoding the contemporaneous CCC data with respective turbo decoders that exchanged information concerning soft data bits to secure coding gain. These various iterative-diversity transmission techniques, although comparatively robust in regard to overcoming additive White Gaussian noise (AWGN), halve available digital payload.
The parallel iterative operation of two turbo decoders consumes more power than is desirable, particularly in battery-powered receivers. Maximal-ratio code combining is a technique that has been used for combining similar transmissions from a plurality of transmitters in multiple-input/multiple-output (MIMO) networks. Searching for a way to avoid parallel iterative operation of two turbo decoders, A. L. R. Limberg considered the use of maximal-ratio code combining of later transmissions of CCC with earlier similar CCC transmissions from the same 8-VSB transmitter. The hope was that a combined signal would be generated that could be decoded by iterative operation of a single turbo decoder. One problem encountered when trying to implement such an approach is that the coding of M/H-service data is not independent of the coding of main-service data in 8-VSB broadcasting per the ATSC standard. The inner convolutional coding of the M/H signal is part of a one-half-rate convolutional coding that intersperses main-service signal components with M/H-service signal components. Accordingly, practically considered, the inner convolutional coding of the later transmissions of CCC and the inner convolutional coding of the delayed earlier transmissions of CCC still have to be decoded separately. The outer convolutional coding of the M/H signal is affected by the pre-coding of the most-significant bits of 8-VSB symbols responding to main-service data interspersed among the most-significant bits of 8-VSB symbols responding to M/H-service data. There are also some problems with measuring the energies of the later transmissions of CCC and the delayed earlier transmissions of CCC to provide the information needed for weighting these transmissions for maximal-ratio code combining.
In a replacement system for DTV broadcasting in the United States of America that uses COFDM of a plurality of carrier waves, the FEC coding of main-service data and the FEC coding of M/H-service data can be kept independent of each other. Also, the inclusion of unmodulated carrier waves among the COFDM carrier waves facilitates measurements of their total root-mean-square (RMS) energy in later transmissions and in earlier transmissions of similar data to provide the information needed to weight later and delayed earlier transmissions appropriately for maximal-ratio code combining.
The reduction in overall code rate that results from repeating COFDM transmissions for iterative-diversity reception can be counteracted by increasing the size of the symbol constellations associated with quadrature amplitude modulation (QAM) of the plural carriers. Increasing the size of the QAM symbol constellations tends to reduce the capability of DTV receivers to decode COFDM transmissions received over the air when accompanied by additive white Gaussian noise (AWGN). FEC coding of data bits is used to facilitate DTV receivers being better able to decode COFDM transmissions accompanied by AWGN. Various types of FEC coding are particularly effective for enabling DTV receivers to overcome AWGN by using iterative decoding procedures called “turbo decoding” because of a fancied resemblance to turbo-charging in automobile engines. The various types of FEC coding that can use turbo decoding procedures are collectively referred to as “turbo coding” in this specification, although the term was originally applied specifically to what is now called parallel concatenated convolutional coding (PCCC). By way of specific examples, turbo decoding procedures are also applicable to serial concatenated convolutional coding (SCCC), to product coding and to parallel concatenated low-density parity-check (LDPC) coding.
Iterative-diversity reception implemented at the transfer-stream (TS) data-packet level does not require as much delay memory for the earlier transmitted data as delaying complete earlier transmissions to be concurrent with later transmissions of the same data. This is because the redundant parity bits associated with FEC coding contained in those complete earlier transmissions is removed during its decoding and so do not need to be delayed. However, implementation of diversity reception at the TS data-packet level sacrifices the substantial coding gain that can be achieved by decoding delayed earlier transmissions concurrently with later transmissions of similar data and interchanging preliminary decoding results between the concurrent decoding procedures. Implementation of diversity reception at the TS data-packet level is also incompatible with code-combining of delayed earlier transmissions and later transmissions of similar data being used to improve signal-to-noise ratio (SNR).
European engineers have updated the COFDM transmissions used in DVB-H standard for European broadcasting so as to support a form of iterative-diversity reception. The orthogonal coordinates of lattice points in 16QAM symbol constellations are rotated so the imaginary-axis coordinates duplicate the real-axis coordinates. Then the imaginary-axis coordinates of successive 16QAM symbol constellations are delayed a prescribed period of time respective to their real-axis coordinates to provide iterative diversity between the two sets of coordinates. The rotation of the axes of the orthogonal coordinates decreases by a factor of four the spacing between lattice-point coordinates along each axis. It is observed here that it is preferable to repeat 256QAM symbol constellations without rotation, rather than using rotated 16QAM symbol constellations. The spacing between lattice-point coordinates along each axis is reduced by a factor of four by going from 16QAM symbol constellations to 256QAM symbol constellations, too. The duplication of the 256QAM symbol constellations halves their digital payload. However, sixteen times as many lattice points are available in each 256QAM symbol constellation as in each 16QAM symbol constellation. So, overall, a pair of the repeated 256 QAM symbol constellations provides eight times the digital payload of the rotated 16QAM symbol constellation of same duration as each of the 256 QAM symbol constellations. This eight times larger digital payload can support more forward-error-correction (FEC) coding, if such be desired.